1 A R C H I V E S O F M E T A L L U R G Y A N D M A T E R I A L S Volume Issue 3 I. JENDRZEJEWSKA, J. MROZIŃSKI, P. ZAJDEL, T. MYDLARZ,T.GORYCZKA, A. HANC,E.MACIĄŻEK X-RAY AND MAGNETIC INVESTIGATIONS OF THE POLYCRYSTALLINE COMPOUNDS WITH GENERAL FORMULA Zn x Sn y Cr z Se 4 BADANIA STRUKTURALNE I MAGNETYCZNE POLIKRYSTALICZNYCH ZWIĄZKÓW O OGÓLNYM WZORZE Zn x Sn y Cr z Se 4 Seleno-spinels with the general formula Zn x Sn y Cr z Se 4 (where x + y + z 3) were prepared as polycrystalline samples in the two nominal compositions: Zn 0.9 Sn 0.1 Cr 2 Se 4 and ZnCr 1.9 Sn 0.1 Se 4, using ceramic method. X-ray powder diffraction was used to analyse the obtained phases and to determine their crystal structure and lattice parameters. The obtained single-phase compounds crystallize in the spinel cubic structure Fd 3m. Tin ions are found to occupy both tetrahedral and octahedral sublattices. Chemical compositions of the obtained samples were determined using JEOL-type Scanning Microscope which also revealed a variation in local distribution of cations and the porosity of the samples. The magnetisation data for Zn x Sn y Cr z Se 4 system, shows that the saturation magnetic moments depend on location of tin ions in crystal lattice of ZnCr 2 Se 4. It was found that the magnetic properties correspond well both with the chemical composition and with the crystal structure. PACS: cp; Fn; Cr; Ee. Keywords: Inorganic materials; Sintering; Magnetic and X-ray measurement Metodą ceramiczną otrzymano polikrystaliczne związki chemiczne o ogólnym wzorze Zn x Sn y Cr z Se 4 (gdzie x + y + z 3) dla dwóch założonych składów Zn 0.9 Sn 0.1 Cr 2 Se 4 izncr 1.9 Sn 0.1 Se 4. Skład chemiczny określono przy użyciu mikroskopu skaningowego JEOL (SE 6480). Dla związku o nominalnym składzie Zn 0.9 Sn 0.1 Cr 2 Se 4 wyznaczono skład rzeczywisty jako (Zn 0.87 Sn )Cr 2.02 Se 4, natomiast dla związku o nominalnym składzie ZnCr 1.9 Sn 0.1 Se 4 wyznaczono skład rzeczywisty jako Zn 0.93 [Cr 1.95 Sn 0.05 ]Se 4. Za pomocą rentgenowskiej analizy strukturalnej oraz metody Rietvelda wyznaczono strukturę i parametry sieciowe otrzymanych związków. Badania magnetyczne wykonane w silnych polach magnetycznych wykazały, że momenty magnetyczne nasycenia zależą od obsadzenia jonów cyny Sn 2+ w sieci krystalicznej ZnCr 2 Se 4. Dla otrzymanych związków (Zn 0.87 Sn )Cr 2.02 Se 4 izn 0.93 [Cr 1.95 Sn 0.05 ]Se 4 zmierzone momenty magnetyczne nasycenia wynoszą odpowiednio 6.52µ B /cz. i 5.56µ B /cz. Wzrost nasycenia namagnesowania w (Zn 0.87 Sn )Cr 2.02 Se 4, gdzie jony cyny Sn 2+ podstawiają się w miejsce niemagnetycznych jonów Zn 2+,świadczyotym,żejonycynySn 2+ mają wpływ na momenty magnetyczne w (Zn 0.87 Sn )Cr 2.02 Se 4. Badania magnetyczne wykonane przy użyciu magnetometru nadprzewodzącego SQUID potwierdziły obecność antyferromagnetycznego uporządkowania w sieci otrzymanych spineli oraz wykazały wpływ jonów cyny Sn 2+ na oddziaływania antyferromagnetyczne w tym układzie. 1. Introduction The parent compound of the studied solid solutions of ZnCr 2 Se 4 crystallises in the MgAl 2 O 4 -type normal spinel cubic structure (space group Fd 3m). Zinc and chromium respectively are tetrahedrally and octahedrally co-ordinated in the cubic close packing of selenium atoms, and the lattice parameter has a value a 0 = Å [1-4]. ZnCr 2 Se 4 spinel shows the p-type semiconducting properties and a helical antiferromagnetic spin structure below T N 20 K with a strong ferromagnetic component evidenced by a large positive Curie-Weiss temperature of 115 K . The magnetic properties result from the near-neighbour ferromagnetic Cr-Cr and the more distant neighbour antiferromagnetic Cr-Se-Cr interactions, and it has already been shown that the system can be easily substituted at various sites [6, 7]. Polycrystalline quater- UNIVERSITY OF SILESIA, INSTITUTE OF CHEMISTRY, KATOWICE, 9 SZKOLNA STR., POLAND WROCŁAW UNIVERSITY, FACULTY OF CHEMISTRY, WROCŁAW, 14 F. JOLIOT-CURIE STR., POLAND UNIVERSITY COLLEGE OF LONDON, DEPARTMENT OF CHEMISTRY, LONDON, UK INTERNATIONAL LABORATORY OF HIGH MAGNETIC FIELDS AND LOW TEMPERATURES, WROCŁAW, GAJOWICKA 95, POLAND UNIVERSITY OF SILESIA, INSTITUTE OF MATERIALS SCIENCE, KATOWICE, 12 BANKOWA STR., POLAND
2 724 nary compounds with spinel structure containing zinc, tin and selenium are not known in the literature. In this paper, we report the synthesis of polycrystalline samples from their binary constituents ZnSe-SnSe-Cr 2 Se 3,and the results of their X-ray and magnetic studies with an aim to show the effect of tin substitution on the cation distribution and magnetic properties of these spinels. 2. Experimental 2.1. Samples preparation Polycrystalline samples were obtained in the two nominal compositions: Zn 0.9 Sn 0.1 Cr 2 Se 4 and ZnCr 1.9 Sn 0.1 Se 4.The compounds with general formula Zn x Sn y Cr z Se 4 (where x + y + z 3) were prepared from the binary selenides (ZnSe, SnSe and Cr 2 Se 3 ). The binary selenides were prepared by annealing stoichiometric mixtures of the elements: Zn, Sn, Cr and Se (all elements were of 5N purity). The mixtures were pulverised in an agate mortar in stoichiometric quantities and sealed in fused silica ampoules evacuated to a pressure of about 10 5 Torr. The mixtures were pulverised in an agate mortar in stoichiometric quantities and sealed in fused silica ampoules evacuated to a pressure of about 10 5 Torr. The samples were sintered three more times at 1073K for 198 hours. After the last sinter, the samples were quenched in water with ice Chemical composition The chemical composition of Zn 0.9 Sn 0.1 Cr 2 Se 4 and ZnCr 1.9 Sn 0.1 Se 4 polycrystals was analysed with a JEOL-type Scanning Microscope (SE 6480). The results are presented in Table 1. Scattered electron images (SEI) (Fig. 1) and backscattered images (BEI) (Fig. 2) were collected at a room temperature. Also the chemical composition was determined using an energy-dispersive X-ray detector (EDX) attached to the Jeol microscope. The chemical composition was calculated as an average value using the data from measurements, which were done on individual 10 grains. TABLE 1 Chemical composition of Zn x Sn y Cr z Se 4 compounds using JEOL-type Scanning Microscope Nominal %weight Measured composition composition Zn Sn Cr Se Zn 0.9 Sn 0.1 Cr 2 Se (Zn 0.87 Sn )Cr 2.02 Se 4.00 ZnCr 1.9 Sn 0.1 Se Zn 0.93 [Cr 1.95 Sn 0.05 ]Se 4.00 Fig. 1. A fracture surface by BEI COMPO showing distribution of elements for (Zn 0.87 Sn 0.48 )Cr 2.02 Se 4
3 725 Fig. 2. A fracture surface by SEI for (Zn 0.87 Sn 0.48 )Cr 2.02 Se X-ray Powder Diffraction X-ray powder diffraction data of polycrystalline samples were collected using a Philips X-Pert (PW3050) diffractometer (CuK α radiation (λ 1 = and λ 2 = Å) over an angular range of 2θ : ) in order to make phase analysis and to determine phase composition and lattice parameters. All measurements were carried out at a room temperature. First, the qualitative phase analysis was carried out. Generally, in all studied sintered compounds the presence of the spinel phase was evidenced (Fig.5). Fig. 3. A fracture surface by BEI COMPO showing distribution of elements for Zn 0.93 [Cr 1.95 Sn 0.05 ]Se 4
4 726 Fig. 4. A fracture surface by SEI for Zn 0.93 [Cr 1.95 Sn 0.05 ]Se 4 a) b) Fig. 5. Results of the Rietveld refinement obtained for: (Zn 0.87 Sn 0.48 )Cr 2.02 Se 4 (a); Zn 0.93 [Cr 1.95 Sn 0.05 ]Se 4 (b)
5 727 The Rietveld refinement was performed with the FullProf program v Windows , using at first the nominal composition for site occupancies. When the chemical analysis results became available, the procedure was repeated using the measured ratio of the elements. The comparison of these two approaches is shown in Table 2 and a clear improvement in the quality of the fit in the second case. The determined lattice parameters and the anion free parameters are consistent between the models. Structural parameters obtained from the Rietveld refinement for Zn x Sn y Cr z Se 4 series TABLE 2 x R F (%) R Bragg (%) R wp (%) χ 2 Zn x Sn y Cr z Se 4 a 0 (Å) Coordinate u of Se Nominal Zn 0.9 Sn 0.1 Cr 2 Se (1) (16) (Zn 0.87 Sn )Cr 2.02 Se (1) (16) Nominal ZnCr 1.9 Sn 0.1 Se (2) (16) Zn 0.93 [Cr 1.95 Sn 0.05 ]Se (3) (16) 2.4. Magnetic measurements Magnetization of the investigated compounds was studied in high magnetic stationary fields (up to 14T) within a temperature range of K using an induction magnetometer. The temperature was measured with a thermocouple of chromel-au+0.07%fe with sensitivity 16µV/K. The measurements were performed using a Bitter-type magnet. The saturation magnetization for the polycrystalline compounds (Zn 0.87 Sn )Cr 2.02 Se 4 and Zn 0.93 [Cr 1.95 Sn 0.05 ]Se 4, calculated per one molecule and per chromium atoms is given in Table 3. Magnetization isotherms for (Zn 0.87 Sn )Cr 2.02 Se 4 and Zn 0.93 [Cr 1.95 Sn 0.05 ]Se 4 are given in Fig. 6. TABLE 3 Comparison of the most important parameters obtained from magnetic experiments for (Zn 0.87 Sn )Cr 2.02 Se 4 and Zn 0.93 [Cr 1.95 Sn 0.05 ]Se 4 with the values for pure ZnCr 2 Se 4 Chemical compound µ eff, T=300K µ sat,t=4.2k µ B /mol µ B /Cr µ B /mol µ B /Cr Θ C W (K) T N (K) c M (K/mol) (Zn 0.87 Sn )Cr 2.02 Se Zn 0.93 [Cr 1.95 Sn 0.05 ]Se ZnCr 2 Se 4 [1-4] Fig. 6. Magnetization isotherms for Zn x Sn y Cr z Se 4
6 728 The magnetic measurements of the powdered samples (Zn 0.87 Sn )Cr 2.02 Se 4 and Zn 0.93 [Cr 1.95 Sn 0.05 ] were carried out over the temperature range K using a Quantum Design SQUID-based MPMSXL-5-type magnetometer (Fig. 7 and 8). The superconducting magnet was generally operated at a field strength ranging from 0 to 5 T. Measurements of the compounds samples were made at magnetic field of 0.5 T. The effective magnetic moment was calculated from the µ ef f = 2.83(χ M T) 1/2 µ B equation. Fig. 7. Temperature dependence of the magnetic susceptibility and inverse magnetic susceptibility for (Zn 0.87 Sn 0.48 )Cr 2.02 Se 4 Fig. 8. Temperature dependence of the magnetic susceptibility and inverse magnetic susceptibility for Zn 0.93 [Cr 1.95 Sn 0.05 ]Se 4 3. Results and discussion The applied preparative method allowed to obtain quaternary compounds of ZnCr 2 Se 4 doped with tin and X-ray phase analysis revealed only the presence of spinel phase. The actual composition showed a lower quantity of the built-in tin than the nominal composition, which
7 729 was caused by tin release from the system during the sintering process. The lattice parameters for spinel phase increase to Å for (Zn 0.87 Sn )Cr 2.02 Se 4 and to Å for Zn 0.93 [Cr 1.95 Sn 0.05 ]Se 4 with doped Sn because ionic radius of Sn 2+ (118pm) is bigger than ionic radius of Zn 2+ (74pm) and Cr 3+ (64pm) . A scanning microstructure analysis confirmed the assumed chemical composition. For example, in the case of a sample with the nominal composition Zn 0.9 Sn 0.1 Cr 2 Se 4 the calculated average value was equal (Zn 0.87 Sn )Cr 2.02 Se 4, and for ZnCr 1.9 Sn 0.1 Se 4 sample, composition Zn 0.93 [Cr 1.95 Sn 0.05 ]Se 4 was measured. This method also pointed out the variation in the local distribution of the elements (Fig. 1 and 3) and the porosity of samples (Fig. 2 and 4). In the spinels with general formula ACr 2 X 4 (where A = Cu, Zn, Cd; X = S, Se, Te) only the chromium ions, which occupy the octahedral sites, possess the magnetic moment. However, the value of this moment depends on the type of the A ions occupying the tetrahedral sites. When A = Zn 2+,Cd 2+ or Cu 2+, chromium occurs as Cr 3+ and possesses a magnetic moment of 3µ B per ion . The local symmetry on this octahedral site leads to a non-degenerate orbital ground state with S=3/2. The lattice built upon the B-site consist of tetrahedra of chromium ions. Each chromium ion is common to tetrahedra which are defined by the position of their six first-near neighbours. The Cr 3+ ion is known to have the largest octahedral site preference energy of all cations (69.5 kj/molk) . The spinel structure is stable only when the second cation can occupy a tetrahedral sites. These relations are clearly seen in the mixed crystals. However, the stability of the spinel structure decreases with an increase in anion polarizability (in the order: O, S, Se, Te) . The atoms become too large to fit in the tetrahedral or octahedral sites of the spinel structure, and crystallization occurs in Cr 3 S 4 -type or related structure. This may be reason why SnCr 2 Se 4 not exist. For ZnCr 2 Se 4, the saturation magnetic moment 5.74µ B lies very close to 6µ B /f.u., which is a predicted value for spin only contribution of two Cr ions in high spin electron configuration 3d 3 (S=3/2) [13-15]. The small decrease is attributed to the covalency effects and hybridisation of chromium 3d orbitals with anion 4p states. Tin ions, built-in into the spinel crystal lattice, affect the values of the saturation magnetic moments. Fig. 6 presents the magnetization in saturation state at liquid helium temperature for various tin substitution for both investigated compounds. For the (Zn 0.87 Sn )Cr 2.02 Se 4, the saturation effect is observed at a little lower fields (above 2T). For Zn 0.93 [Cr 1.95 Sn 0.05 ]Se 4 compound, the saturation effect is observed at higher fields (above 6T). For the (Zn 0.87 Sn )Cr 2.02 Se 4 and Zn 0.93 [Cr 1.95 Sn 0.05 ]Se 4 compounds, the measured magnetic moments in saturation are 6.52µ B /f.u. and 5.56µ B /f.u., respectively. Since tin ions substitute here non-magnetic zinc, the increase in saturation magnetisation indicates that Sn2 + may influence on magnetic moment. The magnetic properties of (Zn 0.87 Sn )Cr 2.02 Se 4 as χ M vs. Tand1/χ M vs. T(χ M being the molar magnetic susceptibilities) dependencies are shown in Figures 7. The effective magnetic moment of 5.52µ B at 300K corresponding with presence of magnetic chromium ions. The susceptibility curve exhibits a maximum at 17K, indicating the presence of the antiferromagnetic ordering in the crystal lattice. The values of the Curie and Weiss constants determined from the 1/χ M = f (T) relation over temperature range K are equal to 3.80 K cm 3 mol 1 and 76K, respectively. The magnetic properties of Zn 0.93 [Cr 1.95 Sn 0.05 ]Se 4 as χ M vs. Tand1/χ M vs. T dependencies are shown in Figures 8. A decrease of the effective magnetic moment of 5.41µ B at 300K corresponds with the decrease in the chromium amount. The susceptibility curve exhibits a maximum at 18K, indicating the presence of the antiferromagnetic ordering in the crystal lattice. Generally, the Zn 0.93 [Cr 1.95 Sn 0.05 ]Se 4 compound shows linear 1/χ M = f (T) behavior in the K range, with C = 3.66 K cm 3 mol 1 and Θ = 72K. The decrease in case of both compounds of asymptotic Curie-Weiss temperature and a Neel temperature (Table 3.) indicates that tin in tetrahedral and octahedral site increases the antiferromagnetic interactions in the system. 4. Conclusions Basing on the investigations carried out, we can draw the following conclusions: Multiple sintering and quenching led to incorporation of tin ions into ZnCr 2 Se 4 crystal lattice, both in tetra- and octahedral sites of the spinel structure, and the obtained single-phase compounds crystallized in the spinel cubic structure.multiple sintering and quenching led to incorporation of tin ions into ZnCr 2 Se 4 crystal lattice (space group Fd 3m). The scanning electron microscopy test confirmed the chemical composition of the obtained compounds, but it also showed a variation in local distribution of the elements. Introduction of Sn to the spinel lattice of ZnCr 2 Se 4 system changes the saturation magnetic moment per chromium atom. The saturation magnetic moments depends on location of tin ions in crystal lattice.
8 730 A increase in values of magnetic moments for (Zn 0.87 Sn )Cr 2.02 Se 4 suggests that the tetrahedral sublattice contains Sn 2+ and Zn 2+ ions, the octahedral sublattice contain only Cr 3+ ions. Acknowledgements This paper is funded from science resources for years as a research project (project No. N N ). REFERENCES  J. Hemberger, H.-A. Krug von Nidda, V. T s u r k a n, A. L o i d l, Colossal magnetostriction and negative thermal expansion in the frustrated antiferromagnet ZnCr 2 Se 4, arxiv: cond-mat/ ,  P.K. Baltzer,H.W. Lehmann,M. Robbins, Insulating Ferromagnetic Spinels, Phys. Rev.Lett. 15, 493 (1965).  A. Menth,A.R. von Neida,L.K. Schick,D. L. M a l m, Magnetic properties of Cu-doped ZnCr 2 Se 4, J. Phys.Chem. Solids 33, 1338 (1972).  T. Rudolf, Ch. Kant, F. Mayr, V. Tsurkan, J. D e i s e n h o f e r, A. L o i d l, Optical properties of ZnCr 2 Se 4 Spin-phonon coupling and electronic onsite excitations, arxiv:cond-mat/ v1,  R. P l u m i e r, Étude par diffraction de neutrons de l antiferromagnétisme hélicoïdal du spinelle ZnCr 2 Se 4 en présence d un champ magnétique, J. Phys. (Paris) 27, 213 (1966).  I. J e n d r z e j e w s k a, Influence of nickel substitution on the crystal strcture of CuCr 2 Se 4, J. Alloys and Comp. 305, 90 (2000).  E. Maciążek, A. Molak, T. Goryczka, Influence of cobalt substitution on structure and electric conduction of CuCr 2 Se 4, J. All. Comp. 441, 222 (2007).  J. R o d r i g u e z - C a r v a j a l, Recent advances in magnetic structure determination by neutron powder diffraction, Physica B 192, 55 (1993).  R. D. S h a n n o n, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Cryst. A32, 751 (1976).  P.K. Baltzer,P.J. Wojtowicz,M. Robbins, E. L o p a t i n, Exchange Interactions in Ferromagnetic Chromium Chalcogenide Spinels, Phys. Rev. 151, 367 (1966).  A. W e i s s, H. W i t t e, Kristallstrukture und chemische Bindung, Verlag Chemie, Weinheim  G. B l a s s e, Crystal chemistry and some properties of mixed metal oxides with spinel structure, Philips Res.Rep. Suppl. 3, 1 (1964).  P. Gibart, M. Robbins, V. G. Lambrecht J r, New ferrimagnetic spinel compositions in the system MCr 2 S 4 x Se x where M=Fe, Co, Mn, J. Phys. Chem. Solids 34, 1363 (1973).  J. K r o k - K o w a l s k i, J. W a r c z e w s k i, T. Mydlarz,A. Pacyna,A. Bombik,J. Kopycz o k, I. O k o ń s k a - K o z ł o w s k a, Magnetic properties of the single crystals of ZnCr 2 x In x Se 4 (0.0< x <0.15), J.Magn.Magn.Mater. 111, 50 (1992).  J. K r o k - K o w a l s k i, J. W a r c z e w s k i, T. Mydlarz, A. Pacyna, I. Okońsk a - K o z ł o w s k a, Magnetic superexchange interactions and metamagnetic phase transitions in the single crystals of the spinels ZnCr 2 x In x Se 4 (0 x 0.14), J.Magn.Magn.Mater. 166, 172 (1997). Received: 10 January 2009.
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